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The Monterey Formation at Pt. Reyes, in the Bodega basin, and Pt. Año Nuevo, in the Outer Santa Cruz basin, consists mainly of porcelanite and black quartz chert interbedded with mudstone, siltstone and dolomite. Diatom biostratigraphic analysis of the two sections has resulted in refined age estimates. The Monterey Formation at Pt. Reyes and Pt Año Nuevo are similar in age and range from approximately 15.0 to 13.4 Ma and 14.7 to 13.7 Ma, respectively. The age of the base of the Pt. Reyes and Pt. Año Nuevo sections strongly contrasts with the age of the base of chert-bearing intervals in the Santa Maria basin, where diatom biostratigraphy suggests an age approximately 2 million years younger than intervals in the north-central California Basins. This distribution implies that the onset of upwelling along the California margin was strongly diachronous, with older chertbearing intervals occurring in the more northerly latitudes. The chert-bearing sections in all three basins correlate with middle Miocene periods of high-latitude cooling. However, penetration of cooler water masses, increased upwelling, and biosiliceous sedimentation during the early middle Miocene were probably highly variable across latitudes, affecting the north-central California margin earlier than the south-central margin.

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